Electromagnetic interference (EMI) is subject to strict regulations. EMI limiting values are defined to minimize harmful exposure and thereby avoid damage to people and other systems. The limiting values may apply, in particular, to the intensities which electronic equipment or systems are allowed to radiate within each frequency range.
Clock generators are generally designed as integrated circuits (ICs), and they are used in electronic systems to output a clock signal. In particular, they are designed as oscillators or frequency synthesizers and output a clock signal having a nominal frequency or mid-frequency. Although clock generators play an important role in maintaining the proper operation of most digital systems, they are also the main sources of electromagnetic interference in electronic circuits.
Therefore, an important goal in the development of digital systems which use clock signal-generating or frequency-generating components in their systems is to reduce or limit electromagnetic radiation.
Among other things, shielding, coatings or special filter components are known for reducing electromagnetic radiation. Due to the increasingly higher power densities, in particular ever higher clock rates as well as increasingly stricter electromagnetic interference regulations, measures of this type, however, are reaching their limits in terms of efficiency and cost.
To limit the peak emissions, spread spectrum oscillators (SSO) are therefore being used more and more. These oscillators spread their clock signals over a broader frequency spectrum and thus limit the peak emissions with regard to the individual frequency ranges. This spread is generally achieved by a frequency modulation having a modulation frequency which lies far below the clock frequency. The modulation signal may have, for example, a triangular form or also another suitable form. Due to SSOs of this type, it is possible, for example, to reduce peak emissions by up to 20 dB. A spread spectrum oscillator is known, for example, from the German unexamined patent application DE 10 2005 013 593 A1.
The cycle-to-cycle jitter common in oscillators, which generally produces even much greater fluctuations than does frequency modulation, and yet is averaged over only a few clock cycles, is superimposed on the frequency modulation.
Frequency modulation is generally achieved by a supplementary circuit; however, it is not possible to directly determine during operation whether this frequency modulation is working properly. Furthermore, it is not possible to easily determine in an electronic system whether an SSO was in fact installed or, for example, if a fixed-frequency oscillator was erroneously installed. In this case, substantial electromagnetic pollution of the environment may occur without it being possible to directly detect this situation in the device, i.e., electronic device.
Although methods and systems are known, in principle, for checking proper frequency modulation of an SSO, for example using FM demodulation and subsequently evaluating the modulation signal, or using a frequency spectrum analyzer. However, measures of this type are complex and require extensive measuring equipment. They may also not be easily adapted to an existing production inspection system.